Array of structs - initialization errors

问题:

I'm creating some data structures here (with MFC), compiling in MS Visual C++ 6.0 (yes, it's old).

struct SOpcodeData
{
    BYTE m_byDataType;
    DWORD m_dwMinValue;
    DWORD m_dwMaxValue;
    WORD m_wRepeat;
};

const BYTE DATA_U8   = 0;
const BYTE DATA_U16  = 1;
const BYTE DATA_U32  = 2;

SOpcodeData MY_BYTE  = { DATA_U8,   0,  UCHAR_MAX,  1 };
SOpcodeData MY_WORD  = { DATA_U16,  0,  USHRT_MAX,  1 };
SOpcodeData MY_DWORD = { DATA_U32,  0,  UINT_MAX,   1 };

This code compiles with no errors or warnings. But when I try to create an array of my struct type...

SOpcodeData foo[] = { MY_BYTE, MY_BYTE, MY_WORD, MY_DWORD, MY_BYTE };

VC6 pops a compilation error for each array element:

device.cpp(78) : error C2440: 'initializing' : cannot convert from 'struct SOpcodeData' to 'unsigned char'

No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

Aparently it is mistaking the whole struct type with the first struct field, which is a BYTE (or unsigned char for those who are not used to MFC).

Tried it on Visual Studio 2010 and it works perfectly. But I need to build it using VC6.

I've tried to explicit cast to struct type inside array initialization, but that's redundant and does not solved anything. Any other ideas?

回答1:

Since you insist on using a compiler with lots and lots of known bugs, you will need an ugly workaround:

#define MY_BYTE_CONTENT  { DATA_U8,   0,  UCHAR_MAX,  1 }
#define MY_WORD_CONTENT  { DATA_U16,  0,  USHRT_MAX,  1 }
#define MY_DWORD_CONTENT { DATA_U32,  0,  UINT_MAX,   1 }

SOpcodeData MY_BYTE  = MY_BYTE_CONTENT;
SOpcodeData MY_WORD  = MY_WORD_CONTENT;
SOpcodeData MY_DWORD = MY_DWORD_CONTENT;

SOpcodeData foo[] = { MY_BYTE_CONTENT, MY_BYTE_CONTENT, MY_WORD_CONTENT, MY_DWORD_CONTENT, MY_BYTE_CONTENT };

回答2:

We've figured out another solution, without using #define: using a constructor to initialize the data structures. Sorta like this:

struct SOpcodeData
{
    SOpcodeData (const BYTE byDataType, const DWORD dwMinValue, const DWORD dwMaxValue, const WORD wRepeat)
      : m_byDataType(byDataType), m_dwMinValue(dwMinValue), m_dwMaxValue(dwMaxValue), m_wRepeat(wRepeat)
    {}

    BYTE m_byDataType;
    DWORD m_dwMinValue;
    DWORD m_dwMaxValue;
    WORD m_wRepeat;
};

SOpcodeData MY_BYTE  (DATA_U8,   0,  UCHAR_MAX,  1);
SOpcodeData MY_WORD  (DATA_U16,  0,  USHRT_MAX,  1);
SOpcodeData MY_DWORD (DATA_U32,  0,  UINT_MAX,   1);

SOpcodeData foo[] = { MY_BYTE, MY_WORD, MY_DWORD };

Thanks to everyone!